Aluminum is a preferred substrate for the production of lithographic printing plates, especially for large printing runs. The metal combines very attractive mechanical properties in strength, hardness and ductility with specific chemical properties, especially hydrophilicity, that compel its selection for most commercial lithographic printing operations. Aluminum sheet is very adaptable to a variety of surface treatment techniques, i.e., chemical, electrochemical and mechanical, for graining the sheet surface. Graining greatly expands the surface area of the metal to promote the metals surface water holding power for repelling oily inks as well as to promote adhesion with the photosensitive coatings that comprise the finished plate.
The overall process for the surface treatment of aluminum lithographic sheet precursor as practiced in the prior art may involve eight or more separate operations including etching, desmuting, graining, anodizing, and the like. The prior art processes are described in a paper by Jen-Chi Huang entitled "The Trend in Aluminum Surface Treatment Technology for Lithographic Printing Plate Application" published in "Proceedings of the Symposium on Aluminum Surface Treatment Technology", Robert S. Alwitt--Editor, Vol. 86-11 (1986), pages 2-18 by The Electrochemical Society. Each chemical or electrochemical operation is followed by a water rinse.
A known method for treating aluminum sheets to produce bases for printing plates requires eight process steps wherein aluminum sheets are pretreated prior to graining by immersion in an alkaline pre-etch solution and then a desmut solution. The sheets are rinsed with water after the alkaline pre-etch and desmut, respectively. The sheets are then grained typically by either chemical, electrolytic or mechanical graining, and rinsed again with water. The sheets are post-etched after graining by immersion in an alkaline post-etch solution and then a desmut solution, followed by a so-called "liquid contact" and anodized in sulfuric acid. The sheets are rinsed with water after each of the process steps. Finally, the sheets are interlayered before coating with photosensitive material.
The foregoing illustrates the plethora of different and repetitive operations employed in the prior art to produce aluminum sheet suitable for use as a substrate for a lithographic plate.
The object of the invention is to provide a process for the production of surface treated lithographic plate precursor that uses neither mechanical nor electrical processes for graining of the aluminum surface, thereby resulting in less costly and more productive aluminum treatment lines.